In many ways, these test sessions reminded me of testing early Macintosh applications in 1986, when interaction designers hadn't yet figured out how to use the mouse and a mid-sized GUI. Now they need to get better at supporting fingers and a tiny GUI. There are plenty of guidelines for the seminar — in fact, I've never failed to derive a rich day's worth of material whenever we test a new generation of user interfaces. The "master guideline" remains the same as in 1986: don't port a UI from an old interface paradigm to a new one. In the past, this meant not slapping a GUI on top of something that was inherently a clunky mainframe flow. Now, it means not adding touch-screen access to a desktop-oriented direct manipulation design — users can't touch as precisely as they can click, so the number of manipulable graphical objects should be much smaller (so that each one can be much bigger).

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On mobile devices, applications are easier to use than websites. (Given the current state of affairs; browser-based sites would be easier to use if designers started following more mobile usability guidelines.)

Why are apps better than sites for mobile? Because the more impoverished the device, the more the design must be optimized for the platform's exact abilities, instead of bowing to a cross-platform common denominator.

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The first conclusion from this finding is that pure download numbers are obviously irrelevant. To measure your app's success, you must measure actual use. And, to assess whether you're really meeting user needs, you must go even further and measure sustained use. If people use something a few times and then give up on it, you have a failed mobile design on your hands.

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The finding that iPhone apps are intermittent-use apps gives rise to many design guidelines. Here, we'll discuss a key guideline from the initial user experience:

• Avoid making users pass through a registration screen as the first step.

In our testing, we saw countless apps that asked users to register before having proven their worth in the slightest. This is wrong. Remember: users start out with a fairly low level of commitment to your app. Unless yours is a truly great app that offers immense value, people won't use it enough to make registration worth their while.

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iPhone apps are obviously a class of user interfaces, so it should come as no surprise that general UI guidelines apply, in addition to the special mobile guidelines. The difference between iPhone apps and desktop apps is that, with the former, these UI guidelines are much more critical because mobile typically implies intermittent use. Thus, the initial hurdles must be very low and easy to jump or users will never get accustomed to using your app.